Most sacrifice outlets work at instant speed, so you can use them any time you have priority, including in response to removal
Even if Vivi isn't on the battlefield, your creatures help to make up the difference by providing token creatures or excess mana generation to make it easier to cast multiple spells a turn
At the start of each players turn, they draw a card from their deck
Opposition Agent Opposition Agent didnt break the format the way people predicted it would, but man it sucks when someone flashes it into play in response to your fetch land or Cultivate
You can even cast it without destroying any creatures at all if youre the one whos dominating that side of the battlefield to take out pesky things like pillowfort effects and stax pieces
There's Ultima, which sweeps up the board, and Squall, SeeD Mercenary, who sweeps you off your feet